Classical optical determinations of Earth rotation (UT1) require several weeks of observations to achieve an accuracy of approx1.5 ms of time, whereas modern techniques have demonstrated an accuracy of 0.7−0.9 ms on timescales of 1−5 days l,2. Here we have probed the limits of the accuracy and time resolution of very long baseline inteferometry (VLBI) determinations of UT1 by conducting a series of 1-h observing sessions using only one baseline. Comparison of the results from the 1-h sessions with corresponding determinations from 24-h multi-baseline sessions establishes that the accuracy of the 1-h determinations is close to 0.1 ms of time. These observations were scheduled as part of the project MERIT (monitor earth rotation and intercompare techniques of observation and analysis3) intensive observing campaign.